1. Field of Invention
The present invention relates generally to the field of seismic data acquisition, in particular but not necessarily restricted to alternative methods and apparatus for acquiring seabed seismic data using permanent seabed seismic data acquisition cables.
2. Related Art
Marine seismic exploration investigates and maps the structure and character of subsurface geological formations underlying a body of water. In so-called seabed seismic data acquisition, a cable containing seismic receivers is deployed onto the seabed from a surface vessel for the period over which seismic data acquisition will last. This may be anywhere from several days to several weeks. One specific application of seabed seismic acquisition is called permanent seabed seismic for time-lapse seismic data acquisition. In this case, the cable is deployed on the seabed—or alternatively trenched in the seabed—for permanent or semi permanent use.
In all seabed seismic surveys to date, permanent and retrievable, the cables that are used have an equidistant spacing between sensor units. Since one and the same cable may be used in many different environments with different spatial sampling requirements, usually a cable is designed to meet the most common and demanding requirements. For example, the newest generation of seabed seismic cables all offer a 25 m spacing between sensor units. However, it is well known that in many cases such a close proximity between sensor units is not needed and a sparser spatial sampling of the seismic wavefield is acceptable without leading to a significant loss in seismic data quality. In certain instances it would be desirable to have some flexibility in seabed cable data acquisition geometry that can be tuned to the survey and subsurface characteristics. This is for several reasons, most importantly standardization in the manufacturing process and ease of processing the seismic data.